1. Field of the Invention
This invention relates to delay-line modulators and more particularly to a device for protecting modulators of this type.
2. Description of the Prior Art
A delay-line modulator is intended to deliver pulses to an impedance which consists in most cases of a vacuum tube such as a klystron, a magnetron, a grid tube.
FIG. 1 is a schematic circuit diagram of a delay-line modulator 10 in accordance with the prior art. This modulator has a delay line 11, the input terminal E of which is connected to a circuit for charging the delay line and a circuit for controlling the discharge of the delay line whilst the output terminal S is connected to the utilization circuit 19 represented schematically by an impedance Ze. The delay line 11 comprises a series of elementary cells constituted by a set of capacitors C1, C2 . . . Cr and coils L1, L2 . . . Lr. The line-charging circuit comprises a direct-current feed source 12, a filtering coil 13, a high-capacitance capacitor 14, a load coil 15 and a diode 16. Discharge of the delay line is produced by a thyristor 17 which is controlled by a control circuit 18. The thyristor 17 can be replaced by a thyratron or, more generally, by a controlled switch.
The operation of the modulator of FIG. 1 is accordingly as follows. Charging of the delay line 11 is carried out by means of the diode 16 whilst the thyristor 17 is not in the conducting state. The delay line is charged and its input terminal E attains after a predetermined time interval a potential in the vicinity of 1V.sub.o if V.sub.o is the voltage of the feed source 12. The time-duration T of this charge is given by the formula: ##EQU1## where L is the self-inductance of the load coil 15
C is the equivalent capacitance of the capacitors C1, C2 . . . Cr. PA0 Le is the equivalent self-inductance of the coils L1, L2, . . . Lr and PA0 Ce is the equivalent capacitance of the capacitors C1, C2 . . . Cr.
At the end of this charging time interval, the thyristor 17 is triggered into conduction by a signal delivered by the control circuit 18, with the result that the delay line discharges into the utilization circuit 19.
After this discharge, the delay line can then be recharged but it is important to ensure that this recharge takes place only when the thyristor 17 is cut-off since, if it were in the conducting state, the feed source 12 would be short-circuited.
In order to avoid faulty operation of this type, steps are taken to ensure that the thyristor 17 is released after discharge of the delay line. To this end, the equivalent impedance Ze of the utilization circuit 19 is chosen so as to be lower than the characteristic impedance Zc of the delay line (negative match), the impedance Zc being equal to .sqroot.Le/Ce, where:
As a result of this choice of relative values of Zc and Ze, a negative voltage is brought to the terminals of the thyristor 17 after a time interval .theta. such that: ##EQU2##
It is this negative voltage which turns-off the thyristor 17.
The diagram of FIG. 2 shows the time-dependent variation in voltage at the input terminal E of the delay line 11.
The diagram of FIG. 3 shows the time-dependent variation in voltage at the input terminal of the delay line when Ze is chosen so as to be higher than Zc, which corresponds to a so-called positive match. The thyristor 17 is not turned-off and this would result in a short-circuit of the feed source 12 if recharging of the delay line were permitted.
In order to prevent this faulty operation, it has been proposed to replace the diode 16 by a thyristor which would be triggered into conduction a certain time after the pulse which triggers the thyristor 17 but the choice of this time interval presupposes that the current flowing within the discharge circuit at this instant is lower in value than the thyristor turn-off current. This is not always the case, especially when the utilization circuit is an electron tube such that the impedance presented by said tube to the modulator can vary, for example as a function of the applied voltage, of its conditions of operation or at the time of an accidental short-circuit.
The object of the present invention is therefore to provide a device for protecting a delay-line modulator which prevents charging of the delay line as long as the thyristor for controlling the discharge is not turned-off.